Riblets refer to a surface geometry or surface structure that brings about a reduction in the frictional resistance on surfaces across which turbulent flow occurs. These surface geometries can be fine ribs, for example, which have a pointy rib tip.
In fluid mechanics, a distinction is made between two different types of flows: laminar flow and turbulent flow. The majority of flows that occur in technical applications are turbulent. Turbulent flows generally have higher friction losses than laminar flows. The increased friction in the turbulent flow is created by momentum transfer transverse to the main flow direction in the boundary layer. The flow directly on the surface across which the flow occurs is dominated by vortical structures, which transport high-energy fluid to the wall surface. This additional momentum transfer as compared to the laminar flow is the reason for the increased wall friction in the turbulent boundary layer.
Riblet structures are typically used on surfaces of vehicles that are exposed to high loads by fluid streams, for example in the field of aviation or also for watercraft. A riblet structure comprises a plurality of ribs or peaks and valleys.
The ribs are disposed so that recessed channels or grooves extend between adjoining ribs. The channels typically extend in a direction along the flow direction of the fluid stream across the corresponding surface.
Riblets are able to reduce turbulent fluid flows, for example in the air or in the water, and support the creation of a laminar flow along the surface, whereby drag along the surface is also reduced.
The channels or grooves prevent transverse movements of the vortices in the turbulent flow. Surface friction can thus be reduced. Since the grooves have a low depth in the range of just a few μ meters, they do not represent increased roughness for the flow.
Care must be taken in the dimensioning of the grooves that these must be designed separately for each velocity range of the vehicle. While the riblets can typically be easily applied to the surface to be treated by way of films or adhesive films, the application of identical films to different vehicles is not readily possible due to the special adaptation of the riblet dimensions to the intended velocity range.
PCT International publication WO 2013/050018 A1 describes a riblet film and a method for producing a riblet film.
German patent publication DE 10 2011 121 546 A1 describes a method for generating a riblet structure on a surface of a structural component. The riblet structure is generated by scanning the surface with laser pulses.